Characteristics And Environmental Controllers Of Soil CO₂ And CH₄ Fluxes Of The Xing'an Larch (Larix Gmelinii) Forest

During the last three decades,emissions and uptakes of major atmospheric greenhouse gases,such as CO₂,CH₄and N₂O,have attracted much attention.Reducing and limiting greenhouse gas emissions have become the focus of research by scientists in many fields,and carbon sink enhancement and emissions reduction have become a global consensus and major goals for the carbon neutralization iniative.The Da(Greater)Xing'anling(Khingan)Mountains of the Inner Mongolia Autonomous Region in Northeast China is a coniferous forest region in the cold temperate zone at high latitudes.It is also a permafrost region sensitive to climate change.In this study,using the greenhouse gas measurement systems(UGGA and SF3000),the Xing'an larch(Larix gmelinii)forest in the Da Xing'anling Mountains was selected for continuous in-situ observations of soil surface CO₂and CH₄fluxes from August 2018 to November2020.Daily,monthly,seasonal and interannual changes in soil surface CO₂and CH₄fluxes were analyzed for understanding temporal dynamics of soil CO₂and CH₄fluxes and their responses to changes in environmental factors and for clarifying their roles in the source and sink effects of soil CO₂and CH₄.Portable greenhouse gas measurement systems(UGGA and PS-3000)were adopted to observe soil CO₂and CH₄fluxes of marshland,grassland,and forest land in the growing season(June to September)in 2020.At the same time,the impacts of division of soil respiration components,forest thinning intensity,rainfall reduction and increase,and simulated nitrogen deposition on CO₂and CH₄fluxes were observed and compared for understanding spatial variations in CO₂and CH₄fluxes and impact mechanisms of different treatments on CO₂and CH₄fluxes.From November to December 2020,data on soil CO₂and CH₄fluxes in the winter were acquired and analyzed for in-depth understanding on changes in soil CO₂and CH₄fluxes during the non-growing season.The research results on dynmical changes of CO₂and CH₄fluxes during the freezing and thawing periods,growing season,and the whole year and their controlling factors and on the impacts of artificial interferences are summarized as follows:1.Characteristics of soil CO₂and CH₄fluxes during freezing and thawing periods(1)Characteristics of soil CO₂flux during freezing and thawing periods(1)Daily variations of CO₂flux.During the freezing and thawing period,the daily rates of soil CO₂emissions displayed a single peak pattern.The peak value was reached from noon to afternoon and emission rates fluctuated greatly.The lowest(nadir)emission rates were observed from midnight to dawn and the rates fluctuated only slightly.The ratio of peak to nadir values is 2 to 3.(2)Monthly variations of CO₂efflux.During the freezing and thawing period,the curve of monthly variations of CO₂effluxes basically showed a"U"shape.During the freezing period from October to November,CO₂emissions gradually weakened from 0.96to 0.04?mol·m^-2·s^-1with small fluctuations.On the other hand,CO₂emission rates gradually increased from 0.07 to 0.71?mol·m^-2·s^-1and fluctuated substantially during the ablation period from April to May.(2)Characteristics of soil CH₄flux during freezing and thawing periods(1)Daily variations of CH₄flux.Soil CH₄fluxes showed obvious diurnal changes during the freezing and thawing periods of the soil and is closely related to changes in soil temperature.The diurnal variation curve of CH₄flux showed a bimodal pattern.The peaks of CH₄uptakes appeared concentratively at night,and soil CH₄uptakes reached the minimum at noon.The study found that during the ground freezing and thawing periods,when diurnal variations of soil temperature were between±10?and soil moisture,3%to6%,it favoredthe absorption of soil CH₄(up to 1.0 nmol·m^-2·s^-1or more)(2)Monthly variations of CH₄flux.During the freezing and thawing periods,the curve of CH₄uptake rates displayed in an"n"pattern.During the freezing period from the beginning of October to the end of November,soil CH₄uptake rates decreased from-1.44±0.29 to 0.12±0.03 nmol·m^-2·s^-1,a decrease of 12 times.During the thawing period from late April to the end of May of the following year,the absorption rates of CH₄increased from 0.11 to 1.25 nmol·m^-2·s^-1,an increase of 11 times.(3)Accumulative soil CO₂and CH₄fluxes during the freezing and thawing periodsDuring the ground-surface soil freezing period,the cumulative CO₂flux was 780kg·ha^-1,of which November accounted for 20.57%;the cumulative CH₄flux was 0.60kg·ha^-1,of which November accounted for 19.49%.During the ground-surface thawing period,the cumulative CO₂flux was 960 kg·ha^-1,of which April accounted for 12.24%;the cumulative CH₄flux was 0.52±0.05 kg·ha^-1,of which April accounted for 11%.The accumulative soil CO₂and CH₄fluxes during the freezing and thawing period were1740±180 and 1.12±0.04 kg·ha^-1,respectively.2.Characteristics of soil CO₂and CH₄fluxes during the growing season(1)Characteristics of soil CO₂flux in the growing season(1)Daily variation of CO₂flux.The diurnal CO₂emission rates showed a single peak pattern,reaching the maximum at noon to afternoon,and the lowest emission rates,at midnight to dawn with small fluctuations.The daily variation of soil CO₂flux is significantly positively correlated with soil surface temperature.(2)Monthly variations of CO₂flux.The monthly variations of CO₂flux showed a significant single-peak pattern.From June to September,soil CO₂flux was 1.26,2.51,3.28,1.97?mol·m^-2·s^-1,and the descending order of CO₂effluxes was that of August>July>September>June.(2)Characteristics of soil CH₄uptake flux during the growing season(1)Daily variation of CH₄flux.Soil CH₄uptake flux showed a significant bimodal diel variation.The absorption flux of CH₄was the highest in the early morning and the lowest in the noon or afternoon.The diurnal variation of soil CH₄was significantly negatively correlated with soil surface temperature.(2)Monthly variation of CH₄flux.Soil CH₄absorption rate had a bimodal pattern.Soil CH₄flux values from June to September were-1.386,-2.069,-1.204,and-1.431nmol·m^-2·s^-1,and the absolute value of monthly CH₄fluxes declined in the order of July>September>June>August and began to decrease monotonously in October.3.Seasonal variations of soil CO₂and CH₄fluxes(1)Seasonal variation characteristics of soil CO₂fluxSoil CO₂efflux showed obvious seasonal variations:high in summer and close to zero in winter(0.04?mol·m^-2·s^-1).Soil respiration contributes the greatest to the soil CO₂flux during the growing season.(2)Seasonal variations of soil CH₄fluxCH₄uptake flux exhibits obvious seasonality,with low absorption in spring and autumn,high absorption in summer,and near zero absorption in winter,or even with emissions(0.12 nmol·m^-2·s^-1).Although CH₄emissions occurred in the winter,the overall performance of Xing'an larch forest soil is characteristic of a methane sink.4.Controllers or influencing factors of soil CO₂and CH₄fluxesFrom April to September,soil CO₂and CH₄fluxes had extremely significant correlations with air and soil(0,5,and 10 cm)temperatures and water vapor concentration,with the highest correlation(R²=0.83;P<0.01)between CO₂efflux and soil temperature at10 cm depth.Regression equations from the corresponding fitting model shows that,when soil vapor concentration dipped down to 0.25%,CO₂efflux reached 0;when air temperature dropped to-19.2?,soil CO₂flux lowered to 0.01?mol·m^-2·s^-1;when soil surface temperature was-17.2°C,CH₄flux approximated to 0.Since the daytime air temperature in the Xing'an larch forest area in winter was generally lower than-20?,thus the winter fluxes of soil CO₂and CH₄in this forest area would be almost zero.In addition,according to the correlation analysis and sensitivity analysis of monthly CO₂flux,air and soil temperatures were the main limiting factors for CO₂efflux in the early and late growth stages.5.Effects of different component treatments on soil CO₂and CH₄fluxes(1)Effects of different component treatments in the growing season on soil CO₂and CH₄fluxes(1)Effects of different component treatments on soil CO₂flux.The diurnal dynamics of CO₂fluxes of soil components were unimodal.The CO₂flux declined in the order as follows:total soil respiration(R_S)>heterotrophic respiration(R_H)>litter removal respiration(D)>root off and litter removal respiration(R_D).Soil temperature had a significant effect on total soil respiration and root-cutting respiration.Through root cutting and litter removal treatment,soil CO₂efflux in the understory in the growing season was reduced by 47%-70%.The contribution rate of each component of CO₂flux is as follows.Soil respiration accounted for 41%of total respiration;living roots,26%;litter,33%.(2)Effects of different component treatments on soil CH₄uptake flux.The daily dynamics of CH₄flux in the soil treated with different components were the lowest before and after noon and the highest in early morning;the monthly dynamics indicated the maximum CH₄uptake in July.The control group CH₄flux was the largest from July to September.The CH₄absorption flux was the lowest in the root-cutting treatment,indicating that the live roots promoted CH₄absorption.Seasonal variations of CH₄flux were all bimodal.Through root cutting and litter removal treatments,CH₄absorption flux during the growing season was reduced by 6%to 23%.The contribution rate of each component to CH₄flux:soil accounted for 73%of the total flux;living roots,24%,and litter,3%.(2)Influence of different treatments on soil CO₂and CH₄fluxes in the non-growing season(1)Influence on soil CO₂flux.From November to December,CO₂fluxes of individual component treatment showed trace emission,and the order of CO₂fluxes of the treatment sites was as follows:control group(R_S)>litter removal(D)>root-cutting(R_H)>root cutting and litter removal(R_D),and the CO₂flux values were 0.134,0.079,0.039,and0.006?mol·m^-2·s^-1,respectively.Soil CO₂efflux in late December was 0.079?mol·m^-2·s^-1,2%of the average value(4.1?mol·m^-2·s^-1)of that in the growing season from June to September.(2)Influence on soil CH₄flux.In November and December,CH₄fluxes of individual component treatment showed trace absorption or emission(-0.058-0.069 nmol·m^-2·s^-1).The overall CH₄flux in the control group was absorption(-0.058 nmol·m^-2·s^-1).The CH₄flux in late December(-0.063 nmol·m^-2·s^-1)was 4%of the average of that in the growing season from June to September(-1.52 nmol·m^-2·s^-1).6.Soil CO₂and CH₄fluxes of different forest land types(1)Soil CO₂and CH₄fluxes of different forest types in the growing season(1)Soil CO₂flux of different forest types.CO₂fluxes of grassland in the growing season was the largest in June,with a daily average of 7.833?mol·m^-2·s^-1;those of marshland was the largest in July,with an daily average of 4.609?mol·m^-2·s^-1.Those of forest land were the largest in August,with a daily average of 7.528?mol·m^-2·s^-1.The monthly average CO₂flux during the growing season were 5.461?mol·m^-2·s^-1(grassland),3.590?mol·m^-2·s^-1(marshland),and 5.059(forest)?mol·m^-2·s^-1.Changes in CO₂flux were well correlated with the change of temperature.(2)Soil CH₄fluxes of different forest land types.In the growing season,for different forest land types,monthly CH₄uptake fluxes were all the highest in grassland(-0.339nmol·m^-2·s^-1),marshland(-0.389 nmol·m^-2·s^-1)and forest(-1.987 nmol·m^-2·s^-1)in July.The average CH₄uptake flux of the growth season of grassland land,marshland and forest land was-0.389,0.066,-1.559 nmol·m^-2·s^-1,respectively.The variations of CH₄fluxes were well correlated with changes in soil surface temperature.(2)Soil CO₂and CH₄fluxes of different forest types in the non-growing season(1)Soil CO₂flux of different forest land types.In the non-growing season from November to December,soil CO₂efflux of each forest land type was very small,and the emission rates declined in the order of grassland(G)>forest land(F)>marsh land(W),and their average fluxes were 0.392,0.134,and 0.062?mol·m^-2·s^-1,respectively.(2)Soil CH₄flux of different forest land types.Both marshland and grassland showed CH₄emission(0.123 and 0.117 nmol·m^-2·s^-1,respectively),and forest land otherwise showed a trace absorption(-0.058 nmol·m^-2·s^-1).7.Spatiotemporal heterogeneity of soil CO₂and CH₄fluxesSoil CO₂and CH₄fluxes have significant temporal and spatial heterogeneity.In the growing season,the closer to the root axis,the greater the soil CO₂and CH₄fluxes.The CO₂or CH₄fluxes at 1 m radially from the base of the tree trunk were more than twice that at 4 m,showing strong spatial heterogeneity.This also showed that the root system in the growing season had important impacts on soil CO₂and CH₄fluxes.However,in the non-growing season,the spatial heterogeneity of soil CO₂and CH₄fluxes was not significantly related with the radial distance from the root centerline or axis.8.Effects of interference on soil CO₂and CH₄fluxes(1)Effects o of thinning intensity on soil CO₂and CH₄fluxesThe effect of thinning intensity on soil CO₂and CH₄fluxes could be extremely complex due to impacts of different thinning intensity,months,daytime,different stand structures,and different space of measured plots,awaiting further studies.(2)Effects of rainfall rudction on soil CO₂and CH₄fluxes(1)Effects of rainfall reduction on soil CO₂flux.Rainfall reduction treatment in the growing season has a strong limiting effect on CO₂emissions,reducing CO₂emissions by more than 28%,especially in the beginning and mid-term of the growing season.However,high-intensity rain reduction treatment at the end of the growing season had a stronger promoting effect on CO₂emissions.(2)Effects of rainfall reduction on soil CH₄flux.Rain reduction treatment in the growing season promoted the absorption of CH₄,and the absorption of CH₄increased by more than 18%.In particular,the low-intensity rain reduction treatment had a stronger effect on the promotion of CH₄emission,and the absorption of CH₄increased by 79%.(3)Effects of rain enhancement on soil CO₂and CH₄fluxes(1)Effects of rain enhancement on soil CO₂flux.The effect of increasing rainfall during the growing season on CO₂emissions was extremely significant.Among them,the promotion effect of low-level rain increase is greater than that of moderate precipitation increase,and the effect was long-lasting.(2)Effect of rain enhancement on soil CH₄flux.Increased rainfall during the growing season has the effect of reducing CH₄absorption.The effect of low rain increase on reducing CH₄absorption reached a significant level within 48 hours;that of moderate rain increase on reducing CH₄absorption was extremely significant within 48 hours.(4)Response of soil CO₂and CH₄fluxes to nitrogen deposition/addition(1)Response of soil CO₂flux to nitrogen deposition/addition.All gradient treatments of simulated nitrogen deposition by fertilization promoted the emission of soil CO₂,and the promotion effect reached more than 47%.However,with the passage of time,the promotion effect increased with fluctuations and then decreased fluctuatingly.(2)Response of soil CH₄flux to nitrogen deposition/addition.Nitrogen fertilization treatments with different gradients restrained the absorption of CH₄in the soil during the same period,and the inhibition effect of high-nitrogen treatments was the most significant.This paper analyzes and discusses the environmental impact of soil greenhouse gas fluxes under different disturbance conditions.This study revealed the dynmics of CO₂and CH₄fluxes in the soil of Xing'an larch forests in the Da Xing'anling Mountains during the growing and non-growing seasons,critical period of ground surface soils freezing and thawing,different treatment methods and different interference modes.The study results can provide a scientific basis for the study of mechanisms of carbon cycle and their effects on changes in carbon source and sink of Xing'an larch forest ecosystem in the high latitude cold temperate zone under a warming climate.